A pigment type ink using a pigment as a coloring agent has an advantage of being excellent in light resistance and water resistance as compared with a dye type ink using a dye as a coloring agent. However, since a pigment is generally insoluble in water, it is necessary to be mixed with a resin called as a dispersant for being stably dispersed in water. Also, as compared with a dye type ink, a pigment type ink as a dispersion is poor in stability as a lapse of time since a pigment settles.
Also, it is pointed out that a pigment type ink generally has a disadvantage of being poor in transparency and coloring. Particularly, when recording on a transmission manuscript represented as an OHP film processed for an aqueous recording liquid, transparency is lowered and coloring is remarkably dull due to light scattering by pigment particles.
It is required for ink jet use to be printed at a high speed, but unlike a dye penetrated into the inside of a recording medium, a pigment stays on a surface of a recording medium and accordingly friction resistance of an image obtained becomes a problem.
In order to improve transparency and coloring, it is necessary to have pigment particles dispersed as finely as possible. Particularly, in case of ink jet use for injecting ink droplets from a minute nozzle, a strict dispersion stability and a highly finely dispersed state are required in order to achieve satisfactory performances such as color reproducibility on a recorded material and injecting property at the time of injecting from a nozzle at a high speed. For this purpose, various elaborations are made. For example, an elaboration of fully adsorbing a high molecular dispersant on a pigment or an elaboration of firmly fixing a high molecular dispersant on a finely dispersed pigment by means of coating a pigment with a resin is studied.
For instance, the following proposals have been made. A pigment and a resin are kneaded to prepare a solid colored compound, and the compound is dispersed in a dispersing medium to improve transparency and coloring (JP-A-11-80,633); a storage stability of a dispersion is improved by using a pigment coated with a cured polymer having a polar group insoluble in a dispersion medium (JP-A-5-247,370); a pigment is coated with a resin swellable in a dispersion medium to prevent clogging or bleeding (JP-A-3-240,586); and an aqueous solution containing a resin, an amine and water are stirred to fully dissolve the resin, and a premixed pigment is subjected to dispersion treatment to stabilize injecting by controlling a non-adsorbed resin amount to at most 2% (JP-A-2-255,875).
In the ink jet use, it is required for improving a printing speed to enhance permeability of an ink composition and to reduce a drying time. However, when simply adding a general penetrant, there is a case that the permeability can not be fully provided due to a relationship between a pigment and other components or a case that the penetrant damages color reproducibility, which adversely affects on an image quality. Therefore, various proposals have been made to solve these problems.
For example, by using an acetylene glycol type and/or polysiloxane type surfactant as a penetrant and using an alkyl ether derivative of a polyhydric alcohol such as ethylene glycol monobutyl ether as an organic solvent, a satisfactory color reproducibility is achieved (JP-A-2002-30,237); and by using an ink containing water, a pigment, a wetting agent and a specific polyoxyethylene alkyl ether acetate, an ink having excellent permeability, drying property and image quality and having no strike through is provided (JP-A-2001-254,036).
However, when preparing an ink having a high permeability and a quick drying property, it has been found by the present inventors that a stable dispersibility of a pigment is sometimes spoiled by an interaction among a dispersing resin, a penetrant and a solvent for preparing the ink.
The present inventors have intensive studied to solve these problems. As a result of the study, it has been found that an excellent ink for ink jet and an excellent pigment dispersion for ink jet having a satisfactory dispersion stability can be obtained without spoiling other performances by using a dispersion having a combination of specific plural resins contained, particularly in the preparation of an ink having a high permeability.
Further, it has been unexpectedly discovered that gloss of a recorded material is improved by the present invention as compared with a conventional pigment type ink. Particularly, it has been found that when using a glossy paper as a medium, the improvement of gloss is remarkable.